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New Theory Concludes That the Origin of Life on Earth-Like Planets Is Likely:
Does the existence of life on Earth tell us anything about the probability of abiogenesis — the origin of life from inorganic substances — arising elsewhere? That's a question that has confounded scientists, and anyone else inclined to ponder it, for some time.
A widely accepted argument from Australian-born astrophysicist Brandon Carter argues that the selection effect of our own existence puts constraints on our observation. Since we had to find ourselves on a planet where abiogenesis occurred, then nothing can be inferred about the probability of life elsewhere based on this knowledge alone.
[...] However, a new paper by Daniel Whitmire, a retired astrophysicist who currently teaches mathematics at the U of A, is arguing that Carter used faulty logic. Though Carter's theory has become widely accepted, Whitmire argues that it suffers from what's known as "The Old Evidence Problem" in Bayesian Confirmation Theory, which is used to update a theory or hypothesis in light of new evidence.
[...] As he explains, "One could argue, like Carter, that I exist regardless of whether my conception was hard or easy, and so nothing can be inferred about whether my conception was hard or easy from my existence alone."
In this analogy, "hard" means contraception was used. "Easy" means no contraception was used. In each case, Whitmire assigns values to these propositions.
Whitmire continues, "However, my existence is old evidence and must be treated as such. When this is done the conclusion is that it is much more probable that my conception was easy. In the abiogenesis case of interest, it's the same thing. The existence of life on Earth is old evidence and just like in the conception analogy the probability that abiogenesis is easy is much more probable."
Journal Reference:
Daniel P. Whitmire. Abiogenesis: the Carter argument reconsidered [open], Int J Astrobio, 2022. DOI: 10.1017/S1473550422000350
(Score: 2) by Immerman on Wednesday October 05 2022, @04:16PM (4 children)
I think you're misunderstanding the argument.
The accepted wisdom is that since we have only a single data point, we can't conclude anything about the likelihood of life arising on Earth-like worlds.
His counterargument is that since we exist, we can infer that it was *probably* easy for life to arise here. Simply because if you look at all the (presumably numerous, potentially infinite) intelligent species in the vastness of the universe, most of them would have arose from worlds where it was easy for life to arise. So pick one species at random (us), and they'll probably be from somewhere where it was easy for life to arise. And that is true regardless of the probabilities involved, except in the extreme case where life is more likely to arise on "hard" planets than "easy" planets are to exist.
Now, you are correct that maybe things like our unusual moon is one of the things that made it easier, so we can't automatically assume that all Earth-like worlds are "easy" for life. However, making such assumptions without evidence because of a merely plausible hypothesis is poor scientific form.
Moreover, even if large tides are important for maintaining volcanic activity to feed early protolife, Earth-like moons around gas giants get similar tidal benefits, and judging by our solar system are likely far more common than Earth-like planets - we have over a dozen, at least two of which, Europa and Titan, look incredibly promising for life.
We also don't really know just how unusual our moon is - we've only got our own solar system for reference. We can barely detect Earth-like planets around other stars, moons are still far too small to detect.
And for that matter we don't even know how unusual life is within our own solar system - we've yet to really look for it. And the preliminary searches we *have* done keep hinting that life may actually be quite common - the few tests for life on Mars have all come back positive (though we've come up with alternate explanations that render those results inconclusive), and we've spotted some promising gasses in Venus's atmosphere as well.
Of course, panspermia within a solar system is actually fairly plausible, so we might find life throughout the solar system, all distantly related to us - in which case we can't assume that life actually arose on Earth. Mars is speculated to have been a warm, wet world back when Earth's surface was only beginning to cool enough for water to exist. And the various gas giant moons may have cooled enough even sooner than that. The fact that life was present on Earth almost as soon as liquid water existed hints that either life can arise very easily, or it arrived here from somewhere else.
Alternately, if we find any life that's NOT related to us (e.g. not DNA based), then that would strongly suggest that life can arise very easily, and the galaxy is probably jam-packed with it.
(Score: 2) by maxwell demon on Wednesday October 05 2022, @05:20PM (3 children)
And my argument is that this counterargument is wrong.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by Immerman on Wednesday October 05 2022, @08:11PM (2 children)
You just repeated my point with different words.
There were never any probabilities attached to "easy" - only that it's easier than "hard", so relabeling them as "very unlikely" and "extremely unlikely" changes nothing.
We know little about *how* easy it was for life to arise here, but we do know that it probably *was* easy, compared to most places.
And given that, it seems reasonable to assume that most planets like ours were similarly "easy" for life to get started on - whatever that means. A big moon is really the only characteristic Earth has that we don't have reason to believe are also common elsewhere. (And we have only very limited evidence from which to conclude that it might be rare)
Meanwhile - we know that life not only arose here, it did so almost as soon as liquid water existed on the surface. Which strongly hints that either it was *very* easy to life to start here, or it originated elsewhere.
Basically - if the odds of life arising are equivalent to rolling a die to get a million 1's in a row - then it *might* happen within the first million throws, but it's far more likely to require countless trillions of throws before it happens. And the safe bet is that it took a fairly "typical" number of throws in our case.
(Score: 2) by maxwell demon on Thursday October 06 2022, @04:46AM (1 child)
So you're trying to get out by redefining words? Sorry, "easy" has a meaning. And that meaning is not "all other options are even harder."
And the point remains that we can't say anything about the probability of finding life on other planets, which was Carter's point. We still can't tell because we don't know the probability life had on earth. So even in the unlikely case that Whitmire really meant it in the way you are now claiming, he's still wrong in telling Carter wrong, just for different reasons.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by Immerman on Thursday October 06 2022, @01:52PM
"Easy" and "hard" are a inherently relative concepts that can easily both be applied to the same task based on context:
To someone playing in a sandbox, moving a few dozen more cubic yards of sand is hard, while to someone operating a river dredge, it's so easy it's barely worth mentioning.
An "easy" Olympic figure skating performance is going to be nearly impossible for most skaters.
And when it comes to creating life - "easy" and "hard" are likely to mean very different things that they do for making bread.